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Telnets overall function is
to define a means by which a user or process on one machine can access
and use another machine as if it were locally connected. This makes
Telnet inherently client/server in operation, like so many other application
protocols in TCP/IP. Usually, the Telnet client is a piece of software
that acts as an interface to the user, processing keystrokes and user
commands and presenting output from the remote machine. The Telnet server
is a program running on a remote computer that has been set up to allow
remote sessions.

TCP Sessions and Client/Server Communication

Telnet is used for the interactive
communication of data and commands between client and server over a
prolonged period of time, and is thus strongly based on the concept
of a session. For this reason, Telnet runs over the connection-oriented
Transmission
Control Protocol (TCP). Telnet servers
listen for connections on well-known
TCP port number 23. When a client wants
to access a particular server, it initiates a TCP connection to the
appropriate server, which responds to set up a TCP connection using
the standard TCP
three-way handshake.

The TCP connection is maintained
for the duration of the Telnet session, which can remain alive for hours,
days, or even weeks at a time. The quality
of service features of TCP guarantee that
data is received reliably and in order, and ensures that data is not
sent at too high a rate for either client or server. A machine offering
Telnet service can support multiple simultaneous sessions with different
users, keeping each distinct by identifying
it using the IP address and port number of the client.

Since TCP is a full-duplex
protocol, both client and server can send
information at will over the Telnet session. By default, both devices
begin by using the standard Telnet
Network Virtual Terminal (NVT) method
for encoding data and control commands. They can also negotiate the
use of Telnet
options to provide greater functionality
for the session. While option negotiation can occur at any time, it
is normal for there to be a burst of such option exchanges
when a Telnet session is first established, and only occasional option
command exchanges thereafter.

With the TCP connection in place
and the Telnet session active, the client and server software begin
their normal jobs of interfacing the user to the remote host. To the
user, the Telnet session appears fundamentally the same as sitting down
at a terminal directly connected to the remote host. In most cases,
the server will begin the users session by sending a login prompt
to ask for a user name and password. The Telnet client will accept this
information from the user and send it to the server. Assuming the information
is valid, the user will be logged in and can use the host in whatever
manner his or her account authorizes.

As mentioned in the
Telnet overview, even though the protocol
is classically intended for remote login, it need not be used in this
manner. The administrator of the computer that is running the Telnet
server determines how it is to be used on that machine. As just one
example, a Telnet server can be interfaced directly to a process or
program providing a service. I can recall years ago using an Internet
server that provided weather information to the public using Telnet.
After using the protocol to connect to that machine, you would be presented
not with a login prompt, but a menu of weather display options. Of course
today, the
Web has replaced most of such facilities,
as it is far better-suited to this type of information retrieval.

Key Concept: Telnet is a client/server protocol that uses TCP to establish a session between a user terminal and a remote host. The Telnet client software takes input from the user and sends it to the server, which feeds it to the host machines operating system. The Telnet server takes output from the host and sends it to the client to display to the user. While Telnet is most often used to implement remote login capability, there is no concept specifically pertaining to logins in the protocol, which is general enough to allow it to be used for a variety of functions.

Use of Telnet To Access Other Servers

The Telnet NVT representation is
used by a variety of other protocols such as SMTP and HTTP. This means
that the same Telnet client that allows you to access a Telnet server
can be used to directly access other application servers. All you need
to do is specify the port number corresponding to the service. For example,
this command will allow you to directly interface to a Web server:

telnet www.someserversomewhere.org
80

You will not receive a login prompt,
but instead the server will wait for you to send an HTTP
Request message, as if you were a Web
browser. If you enter a valid request, the server will send you an HTTP
Response message. Used in this way, Telnet
can be very valuable as a diagnostic tool.

Key Concept: The Telnet Network Virtual Terminal (NVT) data representation has been adopted by a host of other TCP/IP protocols as the basis for their messaging systems. Telnet client software can thus be used not only to connect to Telnet servers, but also to servers of protocols such as SMTP and HTTP, which is useful for diagnostic purposes.

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